This application is based on French Patent Application No. 01 04 255 filed Mar. 29, 2001, the disclosure of which is hereby incorporated by reference thereto in its entirety, and the priority of which is hereby claimed under 35 U.S.C. xc2xa7119.
1. Field of the Invention
The present invention relates to a filter and to an antenna including the filter, which antenna can in particular be used in a mobile telephone network.
2. Description of the Prior Art
A telecommunication antenna sends and receives radio waves at frequencies specific to a telecommunication system using the antenna. Thus an antenna for the Global System for Mobile communications (GSM) sends and receives waves whose frequencies are in the 870-960 MHz band.
FIG. 1 shows an installation which includes a GSM base station 10 and a GSM antenna 14. A base station is usually at ground level, for ease of maintenance, whereas an antenna is usually high upxe2x80x94on a pylon, water tower, etc.xe2x80x94to maximize its send and receive coverage area. For this reason the station 10 is connected to the antenna 14 by cables 16 transmitting radio waves between them.
Various forms of electromagnetic interference, due to waves sent by another antenna, for example, degrade the waves transmitted in this way. Also, the waves produced by the station 10 may include unwanted frequencies outside the GSM frequency band. A filter 12 is therefore placed between the base station 10 and the antenna 14. The filter 12 processes the waves transmitted by the cables 16 to attenuate those whose frequency is outside the band used by the antenna 14. The filter 12 is an air filter, for example, formed by a hollow enclosure with metal walls whose dimensions are such that waves at particular frequencies are attenuated by resonance as they propagate in the enclosure.
Locating filters outside the antennas has many drawbacks. The cables used in these installations are costly. The quantity of cable used is increased by locating the filters outside the antennas. Also, manual connection of the cables to the filters leads to additional costs and the risk of damage to the cables and the filters. Using cables between the filters and the antennas degrades the waves transmitted by the cables, because of transmission losses or external interference due in particular to signals radiated by other antennas. This is undesirable, especially for the waves sent to the antenna, because they are not filtered afterward.
U.S. Pat. No. 6,201,801 describes a single-band antenna in which a single send/receive filter is disposed inside the chassis or housing containing the radiating elements of the antenna.
Multiband antennas including radiating elements used for respective different telecommunication systems are known in the art. A multiband antenna of this kind requires filters, but producing filters incorporated into the same chassis or housing as the antenna is particularly difficult, because of the size of the filters. For example, in a multiband antenna including GSM radiating elements using the 870-960 MHz band and radiating elements for the Digital Cellular System (DCS) using the 1710-1880 MHz band, it is necessary to provide a GSM filter and a DCS filter respectively connected to the GSM radiating elements and to the DCS radiating elements.
The object of the invention is to propose a microwave filter that can easily be incorporated into a multiband antenna.
The invention provides a microwave filter including a transmission microstrip, at least one lateral microstrip connected to the transmission microstrip, and at least two dielectric resonators, and wherein said at least one lateral microstrip is coupled to said at least two dielectric resonators so that it can resonate with said at least two dielectric resonators.
The above filter enables filters to be incorporated into the chassis or housing of an antenna because the collaboration of at least two resonators with the same microstrip provides a filter which, for the same performance, is more compact than a combination of independent filters each including a dielectric resonator collaborating with a single lateral microstrip.
In a preferred embodiment, the lateral microstrips form a series of U-shapes, two successive U-shapes having a common branch.
In a particular embodiment, the center of each dielectric resonator is equidistant from two branches of a U-shape.
In a preferred embodiment, each dielectric resonator has a relative permittivity of not less than 10.
The filter advantageously further includes adjustment elements adapted to be moved arbitrarily relative to the dielectric resonators to modify respective resonant frequencies of the dielectric resonators.
In a preferred embodiment, each lateral microstrip has a length substantially equal to 3xcexm/4 where xcexm represents a wavelength to be attenuated.
The invention also provides a microwave antenna including radiating elements and at least one filter as defined above in a common chassis or housing.
One embodiment of the antenna includes radio frequency protection for the filter.
Other features and advantages of the invention will become apparent from the description of embodiments of the invention given by way of non-limiting example and with reference to the accompanying drawings.